Just went through a cap & nipple fight my self, was not going to have to use 2 different size caps.
You may want to get a new set of nipples for the older gun.
What I did was to chuck up the over sized nipples in a drill press (poor man's lathe) and turn them down with a safe-sided file, also used a triangle file. I measured them often and spun a marker on them to keep track of where I was filing.